Streptogramins form a homogeneous group of antibiotics, consisting of a combination of two types of molecules which are chemically different; on the one hand polyunsaturated macrolactones (A-group components, two examples of structures of which are presented in FIG. 1), and on the other hand depsipeptides (B-group components, three examples of the structure of which are presented in FIG. 2). This group comprises many antibiotics (see Table 1 and FIG. 3), which are known by different names in accordance with their origin, including pristinamycins, mikamycins and virginiamycins (for a review, see Cocito 1979, 1983).
The A and B components have a synergistic antibacterial activity which can reach 100 times that of the separate components and which, in contrast to that of each component, is bactericidal (Cocito 1979). This activity is more especially effective against Gram-positive bacteria such as staphylococci and streptococci (Cocito 1979, Videau 1982). The A and B components inhibit protein synthesis by binding to the 50S subunit of the ribosome (Cocito 1979; for a review, see Di Giambattista et al. 1989).
Streptogramins are chiefly produced by actinomycetes, including many streptomycetes, presented in Table 1. In addition, streptogramins are also synthesized by eukaryotes such as Micromonospora which synthesizes vernamycins. Actinomycetes constitute a very important group of microorganisms on account of the large amount of secondary metabolites they produce, including many antibiotics (beta-lactams, tetracyclines, macrolides, aminoglycosides, polyacetates and the like), herbicides, anticancer agents, antifungal agents, immunomodulators and enzyme inhibitors. Many biosynthesis pathways relating to antibiotics belonging to miscellaneous classes as well as other secondary metabolites such as pigments (for a review, Chater 1990) have already been studied at the present time in actinomycetes. An important aspect of this group of bacteria is that the genes involved in the same biosynthesis pathway, structural genes and also resistance gene(s) and regulatory gene(s), are grouped together physically on the chromosome, constituting clusters which can reach more than 100 kb (Hopwood et al. 1986a, Hopwood et al. 1986b, Hallam et al. 1988, Anzai et al. 1987, Ohnuki et al. 1985). To date, no example has been found to contradict this observation. Such a structural organization is of great interest in the development of strategies for cloning biosynthesis genes. In effect, it is possible, starting from a single gene previously cloned by various techniques, a biosynthesis, resistance or regulatory gene, to walk along the chromosome and thus to isolate the set of genes of the biosynthesis cluster.
Our knowledge of the biosynthesis pathways of each of the components of streptogramins is still very incomplete, but the origin of the different parts of each molecule has been identified by radioactive labelling (Kingston et al. 1983). Thus, the A-type components are made up of two regions originating from the condensation of acetates and several amino acids such as serine and glycine, for example. As regards the B-type components, studies have shown that all the amino acids present in the peptide chain are derived from natural amino acids (Hook and Vining 1973). However, no polypeptide involved in these pathways has, to date, been purified in sufficient amounts to permit its molecular characterization, and no biosynthesis gene has been described. In the process of biosynthesis of the B-type components, two parts may be distinguished:
1) Synthesis of the precursors, or of their analogues, of the macrocycle: 3-hydroxypicolinic acid (3-hPic), L-2-aminobutyric acid (L-Abu), p-dimethylamino-L-phenylalanine, 4-oxo-L-pipecolic acid (4-oPIP), L-phenylglycine (L-Phg).
2) Formation of the macrocycle from the precursors mentioned above, L-threonine and L-proline, or their analogues, with possible modification of these precursors or peptide N-methylation.
To date, only the probable metabolic origin of the precursors of the macrocycle of the B-type components has been determined by studies using labelled isotopes (Reed et al., 1986, Molinero et al., 1989, Reed et al., 1989).